Unlike other organic resins, silicone resins have superior heat resistance, weather resistance, water resistance, and flame retardance and are capable of forming a high hardness surface. Then curable silicone resins having crosslinkable groups such as alkoxy and silanol groups attached to silicon atoms in a molecule are widely used in a variety of fields and applications including surface protective materials, heat resistant paint, weather resistant paint, water repellents, and binders. In the recent years, the range of their application is expanding as evidenced by their use as a binder for a hydrophilicity-imparting coating composition intended for preventing surface staining, and their use under study as a low dielectric constant material for forming interlayer dielectric film.
Inter alia, silicone coating compositions are used as coating agents which are coated to various articles such as daily commodities, industrial instruments, and various equipment for traffic facilities, including furniture, fittings, floors, wood, stone, metal plates, building materials, electric appliances, automotive exteriors, interiors and exteriors of residential houses, buildings and concrete structures for providing protection to the surface of the articles. The surface protection with silicone coating compositions is intended for the prevention of mars and flaws, the prevention of corrosion, prevention of stains, the prevention of degradation by ultraviolet radiation, sea water, wind and weather, and improvements in outer appearance or luster.
Of such silicone coating compositions, solutions of curable silicone resins having terminal silanol groups and an average molecular weight of about 3,000 to about 2,000,000 in organic solvents such as toluene and xylene, generally referred to as silicone varnish solutions, are most often used in the art. These silicone varnish solutions can form coatings having the excellent properties of surface hardness, adhesion, heat resistance, weather resistance and water resistance, although they still suffer from the following problems.
(1) Organic solvents having a low flash point are essentially included.
(2) Since dehydrating condensation/crosslinking reaction between silanol groups is utilized, a long term of heat curing at a temperature of at least 150° C. is generally necessary to form a coating. As a result, the type of applicable substrates is limited, a vast amount of energy is needed for curing, and the manner of application is limited to the in-line application process, so that the in situ application is essentially impossible.
(3) Although the curing temperature can be lowered to some extent by the combined use of a crosslinking agent and a curing catalyst for promoting crosslinking reaction, the shelf stability consideration requires that the composition be of two part type wherein these components are added and mixed immediately before application.
Under these circumstances, there is a need for a solventless, room temperature vulcanizable silicone coating composition of one part type which is free of an organic solvent, curable at room temperature, and stable during shelf storage. To this end, the use of a silicone alkoxy oligomer having a relatively low molecular weight obtained through partial (co)hydrolytic condensation of an organoalkoxysilane(s) is under consideration. Further investigations have been made on curing catalysts that can effectively promote hydrolytic reaction with moisture and alcohol-removal condensation reaction of this silicone alkoxy oligomer for forming a coating crosslinked through siloxane bonds.
As one solution to the above problems, the applicant previously proposed a coating organopolysiloxane composition as set forth in JP-A 60-233164. This composition is essentially composed of three components: a partially hydrolyzed oligomer of an alkyltrialkoxysilane, a mono- or di-functional alkoxysilane, and an organometallic compound, typically an aluminum chelate compound. The resulting organopolysiloxane composition for coating is free of an organic solvent and remains stable during shelf storage. When applied to a substrate, the composition cures at room temperature into a coating having a high hardness, good substrate adhesion, and weather resistance. Regrettably, it takes a relatively long time of about one hour until the composition becomes tack-free after application. The composition has poor recoat property in repairing the once coated surface. It would be desirable to have a fast-cure organopolysiloxane composition.
For reducing the cure time at room temperature, the applicant proposed a coating resin composition as set forth in JP-A 3-64380. This composition is essentially composed of three components: an acid having at least two hydrogen atoms as acid groups in a molecule, such as phosphoric acid and/or an anhydride thereof, an epoxy group-containing alkoxysilane and/or a partial (co)hydrolyzate thereof, and an organosilane containing at least two alkoxy groups and/or a partial (co)hydrolyzate thereof. The composition becomes tack-free within a short time of 10 minutes or less at 25° C., forming a coating having a very high hardness, water resistance and solvent resistance. In this composition, the epoxy group-containing and epoxy-free silane compounds or partial (co)hydrolyzates thereof are essentially constructed of trifunctional silane units, and the partial (co)hydrolyzate of alkoxysilane containing a number of active silanol groups is used without further processing. Thus, the combined use of an alcohol component such as isopropyl alcohol is essentially required for the purpose of holding down thickening and gelation after mixing of various components. Even so, the drawback of storage instability is unavoidable as is found a thickening with the passage of time during storage. The composition must thus be of two part type. There is a need for a further improvement.
Curing catalysts commonly used for the aforementioned curable silicone resins having crosslinkable groups such as alkoxysilyl and silanol groups include organic amines such as triethanolamine; organic amine salts such as dimethylamine acetate; quaternary ammonium salts such as tetramethylammonium hydroxide and organosilicone quaternary ammonium salts; alkali or alkaline earth metal salts of organic acids such as sodium hydrogen carbonate and sodium acetate; aminoalkylsilane compounds such as γ-aminopropyltriethoxysilane and N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane; metal salts of carboxylic acids such as iron octylate and zinc octylate; organotin compounds such as dibutyltin dilaurate, dibutyltin dioctate and dioctyltin dilaurate; titanates such as tetraisopropyl titanate and tetrabutyl titanate; and organoaluminum compounds such as acetylacetone aluminum salt. Simply adding these curing catalysts is difficult to solve all the problems.
In this regard, it has been considered to combine curing catalysts of different types. JP-A 10-60377 discloses a coating composition comprising four components: a silanol group-containing polyorganosiloxane, a glycidoxypropyl group-containing alkoxysilane, a difunctional alkoxysilane, and a curing catalyst which is a mixture of an organotin compound and a carboxylic acid amine salt. However, its curing time at room temperature and coating hardness are below the satisfactory level, and the composition is still short in storage stability.
JP-A 2002-356652 discloses a coating composition comprising three components: a silicone oligomer containing silanol and alkoxy groups, a curing catalyst which is a mixture of a metal chelate compound, a volatile acid and an amine-derived silane coupling agent, and a solvent mixture of a specific ester, ketone and ether. This composition has problems including the presence of organic solvents, a prolonged curing time for coatings of increased thickness, and difficulty to form fully hard coatings.